How to recognize signs of life on Earth and other worlds

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How to recognize signs of lifeon Earth and other
worlds
Wesley A. Traub Harvard-Smithsonian Center for
Astrophysics

• Astrobiology Course for Middle and High School
  Educators
• Cambridge, Mass., 15 November 2004

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How can we search for life?

• Go we could go there and find them
• Wait we could wait for them to find us
• Look we could look for them from home

The goal of this talk is to show that the colors
(or spectrum) of a planet can tell us what kind
of planet it is, and also if life is likely to
be present there.
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Go 1 We could go there and find them
We can go to Earth, Mars, etc. Look for actual
plants, animals, bacteria. Look for fossil
plants, animals, bacteria. Viking Lander and
Mars Rover did this on Mars. They found no signs
of life. (They found evidence of past water, but
no past life.)
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Go 2 Could we go to the nearest stars?
The nearest stars are about 10 parsecs, or 30
light-years distant. We would have to travel at
the speed of light for 30 years (no!), or 1/10
the speed of light for 300 years (still hard), to
get there. For comparison, the moon is only 1
light-second away, and the Sun is 500
light-seconds distant. So, going to the nearest
stars is very hard today.
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Wait 1 Could we wait for them to find us?
The Hollywood movies and National Inquirer
method they could send ET or aliens to
Earth. Is this likely? If this were happening
today, astronomers and other sky-watchers would
probably have seen them coming. Its possible,
but extremely unlikely. Also, recall that the
nearest stars are very far away!
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Wait 2 Could we wait for their signals to find
us?
A technological society might leak radio, TV,
microwave, and laser light. These signals would
travel through space, and we might eventually
detect them. With a large receiving antenna, or
a telescope, we could easily detect such
signals. No signals have been found yet, but
they might be found if we wait long enough, or
look in the right direction.
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Look 1 Could we look for them from home?

• We could look for Earth-size planets, hoping that
  they might
• have life on them.
• There are several indirect ways to look for these
  planets
• Radial velocity About 140 giant planets have
  been found. This method is not sensitive enough
  to find Earths.
• Astrometric motion Search for wobble,
  left-right, up-down. No discoveries yet.
• Occultation Search for planet in front of star,
  slight dimming.
• Two discoveries so far.
• Gravitaional lensing Search for brightening of
  background galaxy. One discovery so far.

Some of these methods will be discussed in later
meetings.
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Look 2 Could we look for them from home?

• We could look for planets directly, and search
  for signs of life.
• Here is one way to do this
• Take a picture of a planet, and look for roads,
  buildings, canals,
• walls, etc. This has been studied, but sounds
  hard. Here is why.
• To just barely distinguish an object from its
  background, we need
• wavelength / lens_size
  object_size / distance.
• So if wavelength visible light, object large
  building,
• and distance nearest stars,
• then the lens size must be as large as the
  distance to the Moon!
• This is not impossible, but it is very hard.

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Look 3 Could we look for them from home?
We could also simply look at the whole planet
itself, and see if its colors show signs of
life. So far, we know of over a dozen kinds of
colors that might tell us whether life is
present. These are called biomarkers. None of
these biomarkers is perfect, but together they
can give us strong circumstantial evidence for
the presence of life.
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Light and color
We know that white light is the sum of various
colors, violet, blue, green, yellow, and
red. Different light sources can have different
colors, so we can think of using color to tell
us about the object we are looking at. In
sunlight reflected from the Earth we see the blue
sky and the green forests, for example. In heat
radiated from the Earth (another form of light),
we can find the temperature of the ground or
clouds.
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A prism or diffraction grating reveals the
colors of light
My office has fluorescent lights with mercury in
them. This diffraction grating photo shows 5
distinct colors.
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Light bulb, seen through a diffraction grating
overexposed
good exposure
Bulb shines through slit in black paper
Blue, green, red spectrum on both sides of light
source
grating materials www.starlab.com
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Sky, seen through diffraction grating
Cut slit in blackened paper, about 1 x 1/16
inch. Stand about 1-2 ft back. Hold camera in
one hand, and grating in other hand. Take
picture. It helps to pre-set camera to a light
scene at about the same distance as slit.
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Hg lamp, seen through diffraction grating
Notice that here we see distinct colors. To the
eye these look like violet, blue, green, yellow,
red. The camera sees these surprisingly well, but
not exactly the same. The distinct colors are
called spectral lines. We can use these to
identify the presence of mercury anywhere in the
Universe.
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Sun, seen through diffraction grating
Black cardboard backdrop makes it easier to see
the spectrum.
Solar spectrum appears smooth here, but with a
narrower slit it would show many spectral
absorption lines. Each line indicates a specific
type of atom.
Sun, partially blocked by leaves and window
frame.
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Comparison of light sources
candle
incandescent bulb
fluorescent lamp
sky
sun
The candle and incandescent bulb have lower
temperatures than the sun. Their spectra are
therefore stronger in the red, and weaker in the
blue. The fluorescent lamp has strong
mercury lines, plus a weak continuum
background. The sky is more blue than the
sun, so has less red light. The sun and sky
both have narrow absorption lines, which would
be seen if the slit were narrower.
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Spectral colors shown as a graph
brightness
0 0.1 0.2 .3 .4 .5 .6 .7 .8
wavelength in microns
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Radio, TV, and visible light family
infrared
FM
TV 2- 6
TV 7- 70
infrared visible (false color)
AM
longer wavelengths
shorter wavelengths
Ref. www.jneuhaus.com
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Reflected light from outer planets and Earth

• Ref. Karkoschka

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Molecule-by-molecule spectra
Infrared
Visible
10?m
2?m
1?m
0.5?m
20?m
5?m
Envelope
all
H2O
O2
O3
CH4
CO2
N2O
Refs Traub Jucks, AGU Geophys. Monograph 130,
2002 Des Marais et al, Astrobiology 2
153 2002.
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Earthshine visible spectrum

• Integrated light of Earth, reflected from dark
  side of moon Rayleigh, chlorophyll, O2, O3, H2O.

Ref. Woolf, Smith, Traub, Jucks, ApJ 2002
also Arnold et al. 2002
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No life life atmospheric abundances

• Gas no life life
• O2 1 100
• N2O 1 100
• CH4 1 1,000
• CO2 1 0.001

Order-of-magnitude values. Other gases not
readily detectable H2, NH3, HCl, CO, N2
Refs Rambler 1989 Margulis Lovelock 1974
Yung DeMore 1999
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Early Earth CO2, CH4, and O2
CO2
O2
CO2
O2 CH4
CH4
Ref. J. Kasting, Sci. Am., July 2004
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Signs of Life Biomarkers
These spectral features can be signs of life
gt oxygen .. from plants gt ozone
. from oxygen gt methane from
bacteria gt red edge chlorophyll
signature gt blue sky . indicates
protective atmosphere gt brightness varies
periodically . indicates rotation gt
brightness varies randomly . indicates
weather gt water . needed for
cells gt carbon dioxide . needed for
plants gt temperature needed for liquid
water These biomarkers are in the visible,
infrared, or both.
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New NASADirectionJanuary 2004
The President set 20 specific goals for NASA,
including Conduct advanced telescope searches
for Earth-like planets and habitable
environments around other stars
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Two TPF missions planned

• TPF-C coronagraph, 8 x 3.5-m mirror
• 2014 launch
• NASA only, at present
• TPF-I / DARWIN interferometer, 3 free-flyers
• 2015 - 2019 launch range
• NASA-ESA collaboration

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TPF Goals

• Detect and characterize Terrestrial planets,
  including a search for biomarkers
• Detect and characterize planetary systems,
  including giant planets and zodi disks
• Carry out general astrophysics investigations

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Solar system at 10 pc (30 light-years)

• Visible
• Earth/sun 10-10
• Infrared
• Earth/sun 10-7

25 mag, 10 billion
17.5 mag, 10 million
visible infrared
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Summary
gt Two missions are planned to search for
Earth-like planets, and search for life on these
planets. gt The 1st mission will be a
coronagraph, a telescope that can block the
starlight and measure the planet light. gt The 2nd
mission will be an interferometer, 2 or 3
telescopes that can search in the infrared, and
measure a spectrum. gt Both missions will use
biomarkers to search for signs of
life. Biomarkers include oxygen, ozone, water,
methane, CO2, chlorophyll, variable brightness,
and a protective atmosphere.
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Coronagraph deployment movie
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Backup slides
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Earth Jupiter-Saturn, 100 stars
Ref. Simulations by Bob Brown, STScI
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Image-plane coronagraph simulation
1st pupil
1st image with Airy rings
mask, centered on star image
2nd pupil
Lyot stop, blocks bright edges
2nd image, no star, bright planet
Measured Airy rings, Kasdin et al
Ref. Pascal Borde 2004 mask is from Kuchner
and Traub 2002.
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Phase ripple and speckles
Polishing and reflectivity errors in pupil
Phase ripples from primary mirror errors
Speckles generated by 3 sinusoidal components of
the polishing errors
No DM
Image plane
With DM
Image plane
Inset experiment. Borde, Traub, and Trauger, 2004
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Shaped-pupil mask
y
v
x
u
Image cut along the x-axis
Pupil Spergel-Kasdin prolate-spheroidal mask
Image dark areas lt 10-10 transmission
A(x, 0) exp(-(?x/?)2) A(0,
y) periodic messy
Ref. Kasdin, Vanderbei, Littman, Spergel,
preprint, 2004
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Continuous-mapped pupil
Input wavefront uniform amplitude.
Mirror 2
100 dB 10-10 25 mag
Output wavefront prolate-spheroidal amplitude.
Output image prolate spheroid
Mirror 1
Compact star image, easily blocked
Refs Guyon, AA 404, p.379, 2003 Traub
Vanderbei, ApJ 2003